1. Field of the Invention
The present invention relates to a fuel cell system and a control method thereof. More specifically, it relates to a fuel cell system provided on a vehicle and a control method thereof.
2. Related Art
Recently, fuel cell systems have drawn attention as new sources of power that can be used to drive vehicles. For example, a fuel cell system can be provided with a fuel cell producing electric power from chemical reactions of reactive gas, a reactive gas supply device supplying reactive gas to the fuel cell through a reactive gas channel, and a control device controlling the reactive gas supply device.
The fuel cell can be structured to include a plurality (e.g., tens or hundreds) of stacked cells. Each cell is configured by holding a membrane electrode assembly (MEA) between a pair of plates. The MEA is configured with two electrodes, such as an anode (i.e. a positive electrode) and a cathode (i.e. a negative electrode), and a solid polymer electrolyte membrane held between these electrodes.
Supplying hydrogen gas as reactive gas to the anode of the fuel cell and oxygenated air as reactive gas to the cathode of the fuel cell, produces an electrochemical reaction from which the fuel cell produces electric power.
In the abovementioned fuel cell system, the amount of air (air mass) in which electric power can be produced in the highest efficiency is predetermined based on the required amount of power production. When the predetermined power production is required, an air mass is supplied to the fuel cell in the amount predetermined based on this required power production.
Incidentally, for stable electric power production by a fuel cell system, hydrogen gas in a reactive gas channel is discharged (hereinafter referred to as “purged”). This purged hydrogen gas is diluted in air in the reactive gas channel, and then discharged (for example, Japanese Unexamined Patent Application Publication No. H07-235324).
In addition, a method for stopping electric power production of the fuel cell by an idling stop to improve fuel consumption of the fuel cell vehicle is disclosed (Japanese Unexamined Patent Application Publication No. 2001-359204).
Incidentally, when a stop command signal for the fuel cell is input by an idling stop during or immediately after purge, the following phenomena occur.
FIG. 6 is a timing chart for until the fuel cell stops after the stop command signal for the fuel cell is input.
As shown in FIG. 6, a current request value is reduced when the stop command signal for the fuel cell is input by idling stop. As a result, current is reduced, and then the speed of the fuel cell vehicle is reduced. With the reduced current request value, the air mass required for electric power production is also reduced.
However, since a purge flag is set in order to perform purge at the same time of idling stop air mass required for diluting hydrogen gas is increased.
Accordingly, there is a problem in that the air mass required for electric power production is decreased by idling stop, but the air mass required for accelerating to dilute the purged hydrogen gas is increased in order to stop the fuel cell system immediately. Thus, the requested value of air mass for the system is more than the air mass required for electric power production. As a result, the air mass wastefully supplied to the system is increased, and then power generation efficiency decreases, whereby fuel consumption is reduced.